Ziegler-Natta catalysts are widely used to produce polyethylene and copolymers thereof. There are many varieties and methods for making Ziegler-Natta catalysts, such as depositing a titanium complex on a solid support such as magnesium chloride and/or silica. Ziegler-Natta catalysts are fairly inexpensive to produce and usually generate polymer products at high levels of productivity.
Typical Ziegler-Natta products have a molecular weight distribution (MWD) greater than about 2.0, more commonly greater than about 3.0, and a melt flow ratio (MFR) defined as I21/I2 ranging from about 24 to about 28. Polyethylene films produced from Zeigler-Natta catalyzed resins are known for excellent toughness and tear properties. To increase production rates when making these polyethylene films it is common to add high pressure low density polyethylene (LDPE) to the Ziegler-Natta catalyzed resins prior to extrusion by dry blending, which reduces the motor load and extruder pressure and can also improve bubble stability for blown films and reduce draw resonance for cast films. LDPE can also be added to linear and substantially linear polyethylene (including polyethylene copolymers) to increase melt strength and suppress draw resonance. Increased melt strength is particularly important in production of thick gauge blown film, especially when a large diameter bubble is required, for example in the production of agricultural films. The suppression of draw resonance is particularly important in cast processes and extrusion coating where, in both cases, the rate of production is often limited by the onset of draw resonance.
The use of high pressure low density polyethylene, however, is detrimental to most physical properties of Ziegler-Natta catalyst resins. For example, the high pressure low density polyethylene in amounts as low as 4 wt % can cause an ethylene-hexene copolymer to assume the properties of an ethylene-butene copolymer. This undesirable consequence can be offset to a certain extent by increasing the gauge of the film and/or lowering the melt index of the Ziegler-Natta catalyzed resin, but both of these approaches counteract the process benefits of adding the high pressure low density polyethylene.
There is a need, therefore, for improved catalysts to produce polyethylene copolymers having a significantly increased molecular weight distribution as expressed by MFR. For example, there is a need for polymer products that have an MFR of greater than about 28, or greater than about 35, that can be used in film compositions without the need for blending in high pressure low density polyethylene.